This application claims priority from Japanese Applications No. 10-45693 filed Feb. 26, 1998, No. 10-45707 filed Feb. 26, 1998, No. 10-45744 filed Feb. 26, 1998, No. 10-45767 filed Feb. 26, 1998, No. 10-127762 filed May 11, 1998 and No. 10-127788 filed May 11, 1998, the contents of which are incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to Synchronous Transfer Mode (STM) communications networks, and in particular to techniques for transferring data with an Internet Protocol (IP) address by way of an STM connection.
2. Description of Related Art
A feature of STM-based circuit-switched networks is that because signals on a physical channel are multiplexed into time slots and circuit-switched, the delay during communication is extremely short and there is no overhead such as the header required in Asynchronous Transfer Mode (ATM) where virtual paths and cells are employed. In a voice call, for example, quality problems arise and echo cancellers may be required if there is a delay of more than 20-30 ms. Not just in telephony, but also in video conferencing and other bidirectional services in general, it is preferable for delay to be short. From this point of view, STM is a transport mode which is well suited to digital telephone networks. However, a conventional STM-based circuit-switched network requires that data are transferred via an STM connection set up in advance from a source user terminal to a destination user terminal. Its applicability is therefore limited and such circuit-switched networks are only being considered for leased data circuits between large businesses.
On the other hand, in data communications based on an IP architecture; by writing the IP address, which is the identification number of a terminal, in the header of a packet and sending this to a network such as the Internet which supports IP, each router successively transfers the datagram without having to search for the destination corresponding to the address in question and without having to set up a connection between the two terminals which want to communicate. Thus no dedicated connection is required and data can be transferred by means of an extremely simple procedure. However, under this scheme delay is longer than in an STM-based circuit-switched network, and some overhead is necessary.
The present inventors have invented, and filed Application Ser. No. 09/205,612, a novel circuit-switched network which combines the advantages of an STM-based circuit-switched network with the advantages of data communications based on IP addresses. In the circuit-switched network disclosed in this prior patent application, burst dataxe2x80x94i.e., a packetxe2x80x94which has been given a logical address such as an IP address, has its logical address analyzed within an STM network and is transferred through the network to the desired destination by means of the destination node E.164 address corresponding to the logical address of the packet""s destination.
The present invention provides a novel circuit-switched network capable of transferring packets via an STM network using a different approach from that disclosed in the patent application outlined above.
A unique feature of the present invention is that it adds a further header to a packet which already has a header in which an IP address has been written, and thereby transfers the packet to the desired destination terminal via an STM network. The additional header contains routing information relating to the route up to the local switch serving the terminal constituting the destination indicated by the IP address, this information having been set in advance by the local switch serving the source terminal. A further feature of this invention is that, during this process, the transit switches relocate an arrived packet from one time slot to a different time slot on the basis of the routing information written in the packet header. It follows that dedicated connections are not set up in advance in the STM network. Instead, each time a switch receives a packet it sets up the time slot in which the packet will be carried at that point in time. As a result, high bit rate data transfer can be carried out as simply and easily as in data communications based on the IP architecture mentioned above, with the entire network acting as if it were an enormous router.
Namely, the present invention provides a circuit-switched network comprising a plurality of local switches each serving at least one terminal, and at least one transit switch connecting these local switches, these local switches and the at least one transit switch being connected via an STM network, wherein each local switch comprises: means for receiving a packet arriving from a terminal, said packet having a header in which an IP address has been written; a table in which is recorded routing information indicating which route should be used for transfer through the STM network to this IP address; means for adding to the packet a header in which is written the routing information corresponding to this IP address, said routing information having been obtained by looking up the table; and means which looks at this added header and transfers the packet in a time slot corresponding to said routing information.
The aforesaid at least one transit switch preferably comprises: means for looking at the header of the packet written in a time slot, the aforesaid routing information having been written in this header; and means for relocating the packet, in accordance with the result obtained by this look-up means, in a time slot corresponding to the routing information written in the header. This routing information is preferably time slot switching information indicating in which time slot the packet is to be written at each transit switch. The time slot switching information is thus information indicating the order of the time slot relocations required when a packet is transferred through the STM network, these relocations being carried out at each transit switch and at the local switch serving the destination terminal.
Preferably, each local switch and transit switch is also provided with means which, if the aforementioned time slot is busy, temporarily stores the packet until the time slot in question becomes idle. This ensures that a packet is not discarded when there is no idle time slot.
The temporary storage means can include means for investigating, at fixed time intervals, whether a time slot is idle or not. It can also include means which, if a time slot is not idle, makes a reservation so that the packet will be preferentially placed in that time slot when the slot becomes idle.
Temporary storage means can be provided for each input line. This avoids the situation where there is no idle time slot on the line used to transfer packets from the input lines to the temporary storage means. It also prevents a high volume of traffic from a particular input line to the temporary storage means hindering packet transfer from another input line to the temporary storage means.
A transit switch can include: means which obtains, from the routing information written in the header of data which has arrived in a time slot, the number of the output line to which this data should be forwarded; means for searching for an idle time slot among the time slots which can be used for an output line with this number; and means which relocates the arrived data in the idle time slot found by this search means.
In this case the search means can comprise means which uses the result of computing a hash function (which treats the output line number as a variable x) as the time slot number corresponding to that output line number. Alternatively, the time slot numbers can run in numerical order, and the output line numbers can be set in correspondence with these time slot numbers. The output line numbers as a whole run in numerical order, but each output line number is repeated the same number of times as the number of time slots allocated for possible use by that output line, and the search means can be provided with a table in which are recorded the output line numbers and the first time slot number of the time slots corresponding to this output line number. Alternatively, the time slot numbers in numerical order can be grouped according to their associated output line number, and the search means can be provided with a table in which are recorded the output line numbers and, corresponding to these output line numbers, the first time slot number of a given group of time slot numbers and the next time slot number after the last number of that group. It is also feasible to provide means for monitoring, for each time slot, whether that time slot is idle or not, and for the search means to comprise a table in which are recorded chains of time slot numbers corresponding to output line numbers, and means which, in accordance with the results obtained by the monitoring means, removes the numbers of busy time slots from the chains and adds the numbers of idle time slots to the chains.
A plurality of routes can be established between source and destination local switches, and the source local switches and at least some of the transit switches disposed at the branch points in these routes can be provided with: a table in which is recorded information mapping these routes to their allocated time slots; and means for looking up this table and selecting one of the routes in accordance with information relating to the destination of the arrived data. This selection means can also include time slot selection means which gives priority to there being few transit switches on the route to the destination.
In this case, the time slot selection means can include means for selecting a time slot corresponding to the route in which there are fewest transit switches on the way to the destination. The time slot selection means can also include means which, when the time slot corresponding to the route with fewest transit switches is being used for another communication, selects the time slot corresponding to the route with the next fewest number of transit switches to the destination. The time slot selection means can also include means which, when the time slot corresponding to the route with the fewest transit switches is being used for another communication, selects a time slot corresponding to another route, irrespective of the advance setting of routes to various destinations.
The temporary storage means preferably includes means for storing the order in which packets are stored, and means which outputs packets in accordance with this order, starting from the first packet that was stored. It can also comprise timing means which begins timing as soon as a packet is stored, and means which outputs a packet when this timing means has reached a prescribed time. The temporary storage means can also comprise means in which a queue is provided for each time slot corresponding to a different route, and which causes a plurality of packets heading on the same route to be stored in a queue of time slots corresponding to this route.
According to this invention, there is provided a plurality of routing information corresponding to a plurality of discrete route sections. Each transit switch can include means which classifies this plurality of routing information into used routing information and unused routing information, and means for selecting the unused routing information from the routing information arriving at the switch. This classifying means can include means which appends to used routing information a bit indicating that the routing information has been used, and means which deletes used routing information.